Metallum Martis, a 1665 book by Dud Dudley, is the earliest known reference to the use of coal in metallurgical smelting. The book is also referred to as Iron made with Pit-Coale, Sea-Coale, &c. And with the same Fuell to Melt and Fine Imperfect Mettals, And Refine perfect Mettals. A significant feature of his great work Metallum Martis is a map showing Dudley Castle where he correctly identifies the order and geographic layout of strata of coal and ironstone under survey.